Chapter 7: Amplifier Selection; Direct Connection Or Constant Voltage; Power Requirements - Toa BS-1030 User Manual

TOA Electronics Speaker Guide

Chapter 7: Amplifier Selection

Direct Connection or Constant Voltage

Installed sound systems commonly use either a direct connection (also called low impedance)
or constant voltage (also called high impedance) amplifier/speaker interface. Direct connection
to a low impedance amplifier allows up to two 8
to series-parallel wiring, which is inadvisable in most situations. To connect more than two speakers
per amplifier channel, most distributed speaker systems use the constant voltage method.
Constant voltage speaker lines (i.e., 70.7 or 25 V) do not actually have constant voltage on
them. Their line impedance is varied using transformers to achieve the same theoretical maximum
voltage in any system. This approach makes it simple to design and scale systems as needed.
Each system uses a step-up transformer on the output of the amplifier to raise its source imped-
ance, and step-down transformers on each speaker to raise their load impedance. The speaker
transformer load impedances are given in Watts (based on the rated line voltage) instead of
Ohms, to simplify system set-up. In addition to making system design and expansion easier,
constant voltage lines also dramatically reduce speaker cable costs, especially in large systems,
by reducing the required thickness of cable for a given distance run. The following sections
demonstrate the design of constant voltage systems.

Power Requirements

NOTE: The term L
Once the number and placement of speakers is decided, calculate the required amplifier power.
First, calculate the power required per speaker using the following steps:
1. Determine the L
adding 6–10 dB above the expected operating level.
2. Find the speaker's sensitivity rating (SPL, 1 W @ 1 m) from the specifications list
Chapter 8: Speaker Application Tables.
3. Use the chart for attenuation over distance (see Figure 5-1 on page 22), find the speak-
er's level at the listener with an input of 1 W (L
chart, find the distance from speaker to listener, then read the amount of attenuation
(dB) from the bottom half of the chart. Subtract this amount from the sensitivity rating
to get the level at the listener with 1 W input (L
4. Subtract L
ference is negative, it is safe to assume that dBW is zero.
will be used denote required level.
req
including headroom for program peaks. Headroom is included by
req
from L to obtain the level increase needed above 1 W (dBW). If the dif-
w req
speakers to be safely driven without resorting
) as follows: In the top half of the
w
).
w
30